Rfi suppression in sar

ABSTRACT

A filter scheme for broadcast interference cancellation that is computationally efficient and numerically robust Airborne Low Frequency Synthetic Aperture Radar (SAR) operating in the VHF and UHF bands has been shown. At least interference Doppler filtering or interference cancellation is utilized. The interference cancellation involves prediction of the interference for each particular reception interval of mixed interference and radar ground response. This prediction is then coherently subtracted from the incoming signal.

FIELD OF THE INVENTION

This invention relates to the field of radar systems co-existing withradio signals for audio and video. More particularly, the presentinvention relates to the field of synthetic aperture radar, SAR,operating in the VHF/UHF range.

BACKGROUND OF THE INVENTION

Due to technology limitations, early radar development had to rely onthe HF/VHF/UHF bands. The ever-increasing demand for frequency bands forradio use has forced some radar applications out of the lower bands.

However, the VHF/UHF bands remain attractive even today where microwavetechnology makes it feasible to operate in high frequency spectra.

Airborne Low Frequency Synthetic Aperture Radar operating in the VHF andUHF bands is becoming an important emerging technology for wide areasurveillance and target detection in foliage. A VHF synthetic apertureradar system denoted CARABAS™ (trade mark?) (Coherent All Radio BandSensing) SAR system has been described in U.S. Pat. No. 4,965,582 andU.S. Pat. No. 4,866,446. An ultra wide band coherent radar system hasbeen disclosed U.S. Pat. No. 6,07,2420.

To obtain sufficient resolution, viz. a few meters or less, theradar-operating band must extend over several tens of Megahertz, i.e.radar reception must be wide band and occur across frequency bandsallocated for radio and TV broadcasting.

The development of airborne multi-octave foliage penetration radars inthe VHF and UHF band renews the discussion on how radar and radio canco-exist in the same frequency band. The cohabitation issue isparticularly pronounced with television broadcast due to its need for alarge bandwidth.

Consider a VHF radar operating in an area of broadcast transmission.Given that radar transmit power is significantly below that of thecommercial broadcast station, the operating range of televisionbroadcast stations and the radar emission is of the same order ofmagnitude and finally that the television subscribers have directionalantennas pointing at the broadcast station, the absolute majority ofsubscribers will receive a television signal, which dominates over theinterference from the radar transmitter with respect to power level. Theradar receiver on the other hand will sense a one-way propagationtelevision interference signal, dominating strongly over thebackscattered radar signal.

The fidelity of analogue television colour reception calls for a verygood signal to noise ratio, enabling typically phase representation towithin 0.5 deg. This would require 40 dB SNR. Assuming PG=1 MW (givendirectivity of television transmitter and receiver antenna), λ=5 m andr=100 km one finds

$P_{{TV}\rightarrow{TV}} = {{{PG}\frac{\lambda^{2}}{\left( {4\pi} \right)^{2}r^{2}}} = {{- 18}\mspace{14mu} {dB}\; m}}$B_(TV) = 5  MHz

Exterior noise at VHF may be 40 dB above thermal noise, viz for

P _(e) =F _(e) kTB=−70 dBm

we find that 40 dB SNR gives a 10 dB margin to exterior additive noise.The cited figures seem characteristic of good receiving conditions andthus the expectation that possible interference from radar should notaffect TV reception quality.

Now, consider interference caused by 20-90 MHz VHF SAR operating on anaircraft at some typical altitude of 10 km. Say that peak radar transmitpower is 1 kW. A linear FM transmit waveform thus radiates 1 kW also inthe TV band. With a 0 dB transmit gain (typical omni-directional lowfrequency SAR antenna) and a 10 dB receiver antenna gain we get PG=10 kWand one has

$P_{{Radar}\rightarrow{TV}} = {{{PG}\frac{\lambda^{2}}{\left( {4\pi} \right)^{2}r^{2}}} < {{- 18}\mspace{14mu} {dB}\; m}}$

Hence, as a worst case, the radar interference and the useful TV signalare of the same order of magnitude. Using a more advanced radarwaveform, in which the TV band transmission is present over the entirelength of the radar transmit signal, the peak power in the TV band canbe reduced by 10 dB. Still compared to the required TV SNR ratio, theinterference level remains high.

Another important aspect concerning cohabitation is the radar dutycycle. The peak power figure presumes a relatively large dutycycle—higher than 10%. The relative time under which the TV reception isinterfered is thus higher than 1%, depending on the waveform and to someextent the interference level as a trade-off parameter.

TV broadcast antennas are vertically extended in order to depress thetransmit signal towards its terrestrial users. For this reason and forthe absence of gain in the radar antenna, one may set PG=10 kW for radarreception of a single TV station. As there may be a number of stationsactive in the spectrum, as a worst case we assume PG=100 kW and getinstead of 0

$P_{{TV}\rightarrow{Radar}} = {{{PG}\frac{\lambda^{2}}{\left( {4\pi} \right)^{2}r^{2}}} < {{- 28}\mspace{14mu} {dB}\; m}}$

As mentioned exterior noise at VHF may be 40 dB above thermal noise,viz. for B_(radar)=50 MHz

P _(e) =F _(e) kTB=−60 dBm

Given the elevated exterior noise temperature, radar transmit power hasto be increased to compensate for this noise. The sited figures forradar transmit power are calculated on this presumption. Giving rise tointerference levels for television reception of the same order as theactual television signal they can hardly be increased. Hence the 30 dBnoise figure of television interference has to be mitigated by othermeans.

The suppression depth of the mitigation step must consequently be 30 dB.There are several basic mitigation techniques

-   -   1. Range spectrum band-pass filtering    -   2. Doppler spectrum filtering    -   3. Cancellation

The first method implemented by narrow-band notching techniques that canbe analogue pre-reception has been explored in connection with the VHFradar work done in Sweden. This option is only feasible if at least thepartial band 20-50 MHz is free from TV-interference (?). Removing asignificant part of the radar spectrum leads to production of sidelobes,which severely degrades radar performance.

The second method is normally associated with the possibility ofremoving carrier waves, letting the radar operate coherently in slowtime by digital post-reception carrier cancellation techniques. Removalof carrier waves is considered inefficient since in the matchedfiltering performed by the SAR-processing, the carrier signal combineswith its modulation leading to an overall increase in the noise floor.

The third method of cancellation presumes that, the TV signal by somemeans can be identified and subsequently subtracted from the combinedradar/television signal. Active noise cancellation is known in the audiofield in which ambient noise is recorded and a mirror signals isprocessed, amplified generated by means of a loudspeaker. The mirrorsignal is generated in a timely and accurate fashion so as to cancel thenoise at a given point. Since the TV signal has many degrees of freedom,and the required cancellation depth is significant, it is doubtful ifsuch a cancellation can be successful in practice. Even the much simplercase of cancelling a pure carrier is by experience a delicate matter.

SUMMARY OF THE INVENTION

It is a first object of the invention to set forth a radar system inwhich VHF and UHF band communication signals, e.g. radio and TV signals,are actively suppressed, so as to enable the radar system be used inareas where such signals are prevalent.

Moreover it is a first object of the invention to reduce radarinterference on communication services such as audio and televisionreception. Although the radar signal power received at the televisionreceiver may only give rise to a small interference content, this maystill be unacceptable. Hence, it is an object of the invention tomitigate the very energy dominating interference of television broadcaston radar reception. On the other hand, mitigating the disadvantageouseffects on radar must occur without requiring increased radar transmitpower, which would aggravate television reception.

Radar is a device for range or distance measurements relying onelectromagnetic echoes. By accurate timing of the interval from themoment of transmission of electromagnetic pulse until the subsequentreturn of the echo, the distance to the object giving rise to echo isestimated, making the fundamental assumption that the pulse propagatesto the object and back linearly and uniformly at the known speed oflight. The accuracy of the range measurement is set by the timingaccuracy ultimately limited by the bandwidth and energy of the pulse.

The last statement is particularly noteworthy, viz. that it is thebandwidth and energy and not the detailed shape of the pulse, which setsthe range resolution. Thus a transmit radar pulse can have a longduration (much longer than the timing accuracy for the echo), and thusconvey a large energy, at the same time spanning bandwidth whichprovides a high range accuracy.

For signals of a duration larger than the timing accuracy, rangedetermination is achieved by signal processing the received signalthrough a transmit signal matched filter, so-called “pulse compression”.The filter can be realised in both the Fourier domain and thetime-domain. In the time domain, such a filter is a correlation betweenthe transmit and receive signal. In the Fourier domain, the filterconsists of multiplying the Fourier transform of the received signalwith the complex conjugate of the Fourier transform of the transmittedsignal. The outcome of this filter is a reflectivity function providingthe radar reflectivity as a function of range.

In the particular application of synthetic aperture radar—SAR—thereflectivity is the mean reflectivity of the illuminated ground alongcircular arcs limited by the radar antenna angular resolution. This meanreflectivity is greatly improved by the SAR principle repeating themeasurement for different positions of the SAR platform, moving over theilluminated ground.

Consider now radar operation, spanning over a band, which encompassesone or several TV broadcast bands. This situation occurs for instance inVHF SAR, operating over may be 70 MHz of bandwidth in which one orseveral TV stations operating across some 5 MHz bandwidth may radiateinto the radar antenna. The TV signals are very strong compared to thereceived radar signal and must in some way be rejected out of thecollected data in order that this will represent the ground reflectivityand thus transform into a good SAR image of the ground.

Consider first that no TV broadcast is present. According to what havebeen stated one may alter the transmit signal from pulse to pulsepreserving the bandwidth and the energy of the pulse. This will notaffect the reflectivity function determined by pulse compression. Thus,in SAR for instance, one may change the transmitted signal in this waywithout affecting the SAR image.

According to a first aspect of the invention, the pulse compression ischanged from PRF pulse to PRF pulse.

In the case of TV broadcast, the received radar signal will also containTV signals from the one or several bands encompassed by the radar band.The analogue TV signals, it is noted, consists to a large degree ofrepetitive messages, viz. each TV frame is quite similar to the next onefor intervals of a second or more, and an TV line will therefore besimilar to the corresponding TV line in the next frame and so on formaybe 50 frames (the frame rate is about 50 Hz). It is clear that if theradar signal was transmitted in synchronism with the TV frames, thepulse compression signal processing will preserve quasi-periodicity ofthe TV signal. After some second of radar operation and data collection,the TV signal will thus appear in the received pulse compressed Fouriertransformed data as a pin spectrum with 50 Hz separation between pinsand may be 1 Hz width of each pin. It will be possible to reject TVsignal by applying a comb filter in the Fourier domain.

According to a second aspect of the invention, the radar signal isoperated in synchronisation with a given TV signal and filtering isapplied for rejecting the TV signal content.

As mentioned above, such rejection principle requires the radar tooperate in synchronism with the TV frame rate. One possibility toachieve synchronisation with the TV signal would be to adapt the radartransmit pulse repetition frequency—PRF—so that very precisely eachradar transmission would occupy a very precise location in TV framereceived onboard the radar platform. In practise, however we mayencounter several stations in which frame synchronism may not beprecisely upheld. In fact the motion of a SAR platform would by itselfinduce a frequency shift (Doppler shift) dependent on the relativelocation of TV transmitter, which prevents a PRF synchronous to allreceived TV bands.

It is noted that a variable PRF is a mere special case of allowing thetransmit signal shape to vary from pulse to pulse. As already pointedout, this can be done without affecting the radar signal. Such variationmay however not only be variable delay corresponding to varying PRF butmay provide the required synchronism to several TV bands within thetransmitted radar band. We now describe how this can be achieved.

The waveform universally used in combination with pulse compression isso-called linear FM, in which a linear frequency modulation is exploitedthroughout the transmit interval. At a large FM rate a certain frequencywithin the radar signal spectrum is reached soon after the onset oftransmission, at a small FM-rate this instance occurs later. Rather thanusing a linear FM-signal one can thus exploit a non-linear signal, whichreaches a certain set of predetermined frequencies within the radarspectrum at certain predetermined times. This non-linear FM signal wouldconsist of linear components, which however are temporally disjunct andadd to form the overall signal. Clearly for this reason such anon-linear FM-signal will act well as a transmit signal.

This type of non-linear FM-signal is directly suitable for the discussedpurpose of establishing synchronism to the TV frame rate. In fact thePRF rate suitable for VHF SAR operation is of the order 1 kHz, whichmeans that the transmit signal is of a duration shorter than 1millisecond. For this type of signal to establish synchronism with oneor several 50 Hz references (the frame rates of different TV bandsencountered) would only require minor adjustments of the FM frame ratesand is certainly without any effect on radar performance whatsoever.

According to a third aspect of the invention, the radar transmit signalis formed as a pulse compressed signal whose characteristic isconstituted by segments of linear FM. Moreover, the pulse compressiontransmit signal is arranged such that for a given TV channel frequency,the time between the radar pulses coinciding in frequency with the TVsignal is an integer divisor to the TV frame period. In this manner theradar interference shows as a constant area in the TV picture over time.

More preferably, the radar signal corresponds advantageously to an equalcomplete number of TV lines. In so far more channels are present in theradar range, the linear segments are arranged such, that the abovecriteria applies for all TV channels.

Practically, the type of ever changing transmit signal required can beobtained by modern arbitrary waveform generator—AWG—technique in whichit is possible to update the AWG memory at very rates consistent withthe PRF-rate. Hence the required adaptation of transmit signal waveformis predicted by a computation based on the received and demodulatedTV-signals and the computed data used to update the AWG.

According to a fourth aspect of the invention, the received radar issignal is improved by predicting a TV signal operating in the radarrange. The radar signal reception is improved by subtracting thepredicted TV interference signal form the received signal.

It is found that there is an important orthogonality aspect of radar andbroadcast transmit signals, which may support their cohabitation: Theinformation in a radar signal (giving the position of radar transmitter)is intrinsic in its spatial covariance properties whereas thecommunication message in a broadcast signal is coded in its temporalcovariance properties.

In effect, the radar transmit signal may change randomly from pulse topulse without affecting its usefulness for the radar application,whereas a communication signal may change randomly from location tolocation without spoiling its information carrying property. To thelatter statement there are two snags. For one, channel equalisation hasto be established so that the stationary transfer characteristics of thereception on any site are compensated for. Secondly, if interferencecomes from a moving radar, transfer properties will change in time,albeit perhaps more slowly than the de-correlation time of thetransferred message. If so, channel equalisation will still be possible.

For a broadcast signal—which is intended for human reception inreal-time—there must be an attribute of permanency. Indeed, an audiomessage must remain stationary for tens of seconds in order to allow alistener to interpret it. A moving image must remain correlated formaybe a second for its contents to be understood.

Accordingly, for each frequency (i.e. TV program) in the broadcast band,the radar transmission adopts the phase and amplitude of the broadcastsignal to be expected incident on the radar at the particular time thisparticular frequency is passed in the transmit chirp. Accordingly, theradar will transmit a copy of the broadcast message and the broadcastreception site will receive two superimposed but time translated (?)versions of the same message. To a TV subscriber, the reception willappear as a broadcast signal that has been made subject to multipathpropagation. In principle—as will be discussed—so-called channelequalisation may bring back the broadcast signal to its original state.

As for the interference of the broadcast signal on radar reception, itseffect is—if the radar transmit signal successfully imitates thebroadcast signal—that it will be stationary on pulse compression. Infact, since pulse compression consists of correlating the transmittedsignal to the received signal, it will in effect contain the correlationƒ(t, τ) between two segments of the broadcast signal separated by fasttime τ and the development of this correlation in pulse-to-pulse timeviz. slow time t. Clearly, if the broadcast signal tends to remainscorrelated over extended periods in slow time, ƒ(t, τ) will tend toremain constant in slow time. Thereby, its spectral support in theDoppler domain will be small. As will be seen, its spread in Doppler mayindeed be much smaller than the SAR Doppler bandwidth and broadcastinterference may be mitigated by bandpass filtering in the Dopplerdomain.

The filtering according to the invention accomplishes a reduction ofinterference energy by the order of 40 dB. It is thereby on parity withthe stochastic noise component, which by its nature cannot be removedfrom the received signal. Consequently, the degrading effect of radiointerference on SAR imagery might be substantially removed.

Further advantages will appear from the following detailed descriptionof the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first embodiment of the radar system according to theinvention,

FIG. 1_1 shows a second embodiment of the radar system according to theinvention,

FIG. 1_2 shows a third embodiment of the radar system according to theinvention,

FIG. 2+3 shows the embodiment of FIG. 1 implemented in a VHF/UHF SARsystem comprising two bar antennas,

FIG. 4 shows a Fourier transform of the radar signal (after node F2 inFIG. 1_1) of the radar signal under strong RFI interference,

FIG. 5 shows the effects of the adaptively filtered radar signalaccording to the invention on a first TV picture signal,

FIG. 6 shows the effects of the adaptively filtered radar signalaccording to the invention on a second TV picture signal seconds afterthe signal shown in FIG. 5, and

FIG. 7 shows a preferred embodiment of the pulse compression accordingto the invention operating in synchronisation with a TV signal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Moving imagery displays both spatial and temporal redundancies, enablingif one wish significant data compression. Unlike the situation fordigital video, no utilisation of this possibility is made in analoguevideo. The spatial redundancy is associated with the psychological factthat in order to be perceived by humans, a video image may only consistof a limited number of spatial features. Hence, for most parts in avideo sequence, any given excerpt of a TV line in a particular framedecays (i.e. change colour along the line) only slowly along the line.Similarly, it is likely that such a part of a TV line will be stronglycorrelated to the corresponding parts in subsequent lines (i.e. temporalcorrelated). The temporal redundancy consists of that given part of animage in subsequent frames that are likely to be strongly correlated. Inparticular there may appear a strong correlation between subsequentframes of the “scene content” of a TV image since it remains relativelyconstant, cf. FIGS. 5 and 6, only interrupted by sudden “scene changes”of a duration of a second or longer.

In FIG. 1, the radar system according to the invention comprises anaerial AL, an input/output switch SW1, a receiving amplifier RX, ademodulator 3 working according to usual demodulation principles for thegiven communication signal prevailing, a demodulation bank 10 containingfrequencies and demodulation forms pertaining to given communicationsignals prevailing in a given geographical range of interest. Accordingto one embodiment of the invention the demodulation bank and thedemodulator corresponds to a usual TV tuner.

In the embodiment shown in FIG. 1, there is moreover provided a firstinput switch S1, an attenuator 14 and a second input switch 15. Thereceiver captures radio interference frames, a sequence of which isexploited to predict the interference component of the combined signalof interference and radar response.

The radar system moreover comprises, a storing unit 4, a prediction unit5, a filter F1, a modulation unit 7 working according to knownmodulation forms pertaining to given communication signals at givenfrequencies, a synchronisation unit 8. In the most simple form, theprediction unit 5, simply repeats the last received picture.

The modulation unit 7 produces a modulated signal, which is a predictionof the next TV frame in the time domain.

According to a preferred embodiment shown in FIG. 1 and also in FIG.1_2, the signal of the modulation unit 7, S1, is subtracted from thereceived combined radar/interference signal in subtract node SB1 toseparate the radar data. In this manner, the received signal is enhancedand the RFI interference is suppressed. The possibility of this schemerelies on the low pass filtering of the predicted interference signal.Because of this, radar data, which are added to interference andsubsequently subtracted, are first filtered to below a few Hertz ofDoppler bandwidth, viz. to the band, which anyway will be notched in thesubsequent 2-dimensional filtering (explained in connection with FIG.3). The radar system moreover comprises an arbitrary wave generating(AWG) unit 9, a transmit amplifier TXL and a 2-dimensional filter (shownin FIG. 3). The generator is operated in synchronisation with thepredicted interference signal, such as a TV signal.

The arbitrary wave generator 9 is adapted for generating an arbitrarytransmit wave at periodical interruptions at a given PRF frequency, e.g.at 30 KHz. The outputted signal may moreover be subject to pulsecompression. Under pulse compression, which is well known in the art,the frequency of the transmitted pulse is increased at a constant rate(Chirp). In the receiver it is introduced in a linear filter thatdecreases linearly with frequency at the same rate (conjugate filtercharacteristic) as the transmitted signal. The matched filtering schemeeffect is that the received pulse.

According to a second embodiment of the invention, a subtraction unit 12and a phase conjugate unit 6 are provided. The conjugate phase unit 6 isconjugate to the transmit pulse compression filter used in arbitrarywaveguide generator AWG 9. The arbitrary wave generator 9 is of astandard type used for instance in coherent VHF SAR.

According to the second embodiment of the invention, the pulse segmentsare established, by means of synchronisation unit 8 and arbitrarywaveform generator 9 for each PRF pulse in a manner, which shall beexplained in connection with FIG. 7. As mentioned above, since thefiltering used in arbitrary waveform generator and in conjugate phaseunit 6 are conjugate, according to principles known from pulsecompression, the signal can be rendered arbitrary without influencingradar operation. This embodiment is further emphasized in the FIG. 1_1embodiment.

The coherency of the interference may be used to separate it from theradar ground response, in which case the interference signal can beregistered essentially continuously. In this scheme, in the demodulationof the interference, the radar response will appear as additional noise,which will be weak compared to the interference. It would thus not upsetthe interference demodulation scheme or the derived phase estimate.

According to a preferred embodiment of the invention,—by predicting thebroadcast signal (repeating at least parts of it) according to the knownmodulation technique pertaining to a given channel—the (return of the)transmit radar waveform, transmitted in each PRF pulse, is modified oradapted to be synchronised with the broadcast TV signals incident on theradar at the moment of transmission.

In FIG. 4, the effects of a typical TV signal in the filter F2 in theradar unit is shown. It shows a relatively constant peak amplitude ataround 0 Hz (30) corresponding to the frame-to-frame stability of 50 Hzand the line-to-line stability of 350 Hz (31). The intermediate peaksvary along the variations in the information content. The level of theradar signal is below the level indicated by line 33.

In FIGS. 5 and 6, which relates to two subsequent analogue TV pictures,the influence of the modified radar pulses according to the invention onthe TV pictures have been shown. It appears that the radar influence onthe TV signal constitutes the same area in the TV picture. The (phase ofthe) radar signal is synchronous with the TV frame periodicity. Theframe-to-frame correlation could typically amount to 0.02 s-1 s. Theradar signal is adapted in such a way that a vertical “band” appearingon the screen occupies the same position on the screen over time.Moreover, preferably the radar interference on the television signalaccording to the invention with the line periodicity of the televisionpicture signal. The line-to-line correlation of the radar signal couldrange between 0.1 ms-0.01 s.

As mentioned above, since the nature of the television signal is suchthat the information content relating to the same area of the screen isoften constant over periods of time, the radar signal can be predictedsuccessfully, simply by repeating the “information content” associatedwith the given area in question based on a reception of the last picturereceived. Accordingly since the radar signal imitates the televisionsignal, the television signal is inflicting a minor influence on theradar reception and is hence rendered to coexist with the televisionsignal. Likewise, the radar signal is inflicting only minor effects onTV reception.

In FIGS. 2 and 3 an implementation of the circuit shown in FIG. 1 hasbeen shown in more detail. FIGS. 2 and 3 relates to a coherent syntheticaperture radar system, denoted Coherent All Radio Band Sensing(CARABAS), of which type for instance has been described in U.S. Pat.No. 4,866,446. The system comprises two bar antennas, AL and AR, mountedon a flying vehicle parallel to the direction of flight for scanning theground to the left and the right side of the vehicle. The circuit ofFIG. 1 is provided in both left and right channels as illustrated byFIGS. 2 and 3.

As illustrated in FIG. 2, the “left” side circuit RX_AL comprises aninput filter F4 . . . , amplifier RX, Analogue/Digital converter, ADC,buffer BFF, first Fast Fourier Transform unit FFT and Right/Leftseparation step 1.

As shown in FIG. 3, the left side circuit RX_AL moreover comprises, asmentioned above subtracting unit SB1, multiplication unit ML1, cornerturn unit CRN_TRN, second Fast Fourier Transform unit, FFT2, filter F2,second corner turn unit CRN_TRN2, second right left separation stepR/L_SEP2, inverse fast Fourier transform unit IFFT and FFB_SAR unitproviding a digital output for further data processing of radar data,such as presentation of SAR data.

In FIG. 7, the manner of adapting the radar pulses in order to bringthem into synchronisation with the TV signal is shown in a schematicexample, where a given TV channel is transmitted at 60 MHz. A first PRFpulse C1 is transmitted at a given linear chirp rate.

For the subsequent PRF pulse, the pulse compression is arranged as twolinear segments C2A and C2B such that for the frequency 60 Mhz, the timebetween the PRF pulse—or the phase—are in synchronisation with theperiodicity, K, of the TV picture frames. Thereby, the major spikes ofTV interference shown in FIG. 4, can be filtered in a comb filter.

It should be understood that even though one major source of radiointerference has been dealt with—namely analogue TV broadcasts—theinvention is applicable to other types of radio interference. One mayalso encounter digital modulation systems. In e.g. OFDM—orthogonalfrequency division multiplexing—the TV signal is de-multiplexed into—forVHF—6000 channels, each holding a 64 place complex amplitude for 1 ms.The amplitudes are read off and put into an IFFT turning them into atime message of 6 MHz bandwidth and 1 ms duration at any suitablecarrier frequency. Clearly there is no difference in principle betweenthis modulation and analogue modulation as regards the possibility ofinterference prediction, but possibly for the fact that for OFDM theexact key to the modulation must be at hand and cannot be “guessed”.

1-4. (canceled)
 5. A radar unit, comprising: an antenna; an arbitrary waveform generator to issue an arbitrary radar waveform signal at a given pulse repetition frequency, the arbitrary waveform generator being adapted to adjust the phase of the radar waveform signal as a function of a phase adjustment signal; a transmit amplifier coupled to the antenna; a receive unit coupled to the antenna to receive multiple radar pulses to retrieve a Doppler spectrum; a 2D filter for generating associate values of radar response and coordinate data; a noise predictor coupled to the receiver to receive at least one prevalent radio frequency interference; a demodulation and decoding bank comprising known information on the modulation and coding principle of the prevalent radio frequency interference signal, the radio frequency interference signal typically operating according to a predetermined refresh frequency at which redundant information is repeated; said noise predictor operative to receive, demodulate and decode the information content of the at least one radio frequency interference signal, wherein: the arbitrary wave generator is adapted to generate pulse-compressed chirps, and wherein the arbitrary wave generator is synchronized with the at least one prevalent radio frequency interference signal, whereby, the arbitrary wave generator is controlled to produce an overall radar pulse composed of at least a first segment and a second segment whose time/frequency rates may differ from one another, whereby the overall radar pulses vary from pulse to pulse such that for the coinciding frequency of the at least one prevalent radio frequency interference signal, the time between radar pulses is an integer divisor of the periodicity of the at least one prevalent radio frequency interference signal.
 6. The radar unit according to claim 5, wherein the overall pulse composed of the first and second segment is formed so in relation to a previous radar pulse, that the frequency range is the same as the frequency range of the previous pulse and the duration of the overall pulse is the same as the duration of the previous pulse.
 7. The radar unit according to claim 5, wherein radar pulses constitute linearly frequency modulated segments of differing time/frequency rates.
 8. The radar unit according to claim 5, wherein the frequency spectrum is divided into a plurality of sub-channels, each sub-channel corresponding to a regulatory radio channel used for one radio or television information source, the radar unit comprising a noise predictor for each radio frequency interference sub-channel overlapping with the radar range.
 9. The radar unit according to claim 8, wherein the overall pulse is formed so that the composite transmit radar pulses varies from pulse to pulse such that for the coinciding frequency of each radio frequency interference signal, the time between radar pulses is an integer divisor of the periodicity of each corresponding radio frequency interference signal.
 10. The radar unit according to claim 5, wherein radar pulses constitute linearly frequency modulated segments.
 11. The radar unit according to claim 10, wherein the radar pulses are temporarily disjunct.
 12. A method of adapting pulses transmitted from a radar unit, comprising: receiving and demodulating at least one prevalent radio frequency interference signal; sensing the periodicity of a component of the at least one prevalent radio frequency interference signal having a given radio frequency interference frequency coinciding with a frequency of the radar pulses; wherein an arbitrary wave generator generates pulse-compressed chirps, wherein the arbitrary wave generator is controlled to produce an overall radar pulse composed of at least a first segment and a second segment whose time/frequency rates may differ from one another, and wherein the radar unit is to receive multiple radar pulses to retrieve a Doppler spectrum, and wherein the arbitrary wave generator is synchronized with the at least one prevalent radio frequency interference signal, whereby the overall radar pulses vary from pulse to pulse such that for the coinciding frequency of the at least one prevalent radio frequency interference signal, the time between radar pulses is an integer divisor of the periodicity of the at least one prevalent radio frequency interference signal.
 13. The method according to claim 12, wherein the overall pulse composed of the first and second segment is formed so in relation to a previous radar pulse, that the frequency range is the same as the frequency range of the previous pulse and the duration of the overall pulse is the same as the duration of the previous pulse.
 14. The method according to claim 12, wherein radar pulses constitute linearly frequency modulated segments of differing time/frequency rates.
 15. The method according to claim 12, wherein the frequency spectrum is divided into a plurality of sub-channels, each sub-channel corresponding to a regulatory radio channel used for one radio or television information source, the radar unit comprising a noise predictor for each radio frequency interference sub-channel overlapping with the radar range.
 16. The method according to claim 8, wherein the overall pulse is formed so that the composite transmit radar pulses varies from pulse to pulse such that for the coinciding frequency of each radio frequency interference signal, the time between radar pulses is an integer divisor of the periodicity of each corresponding radio frequency interference signal.
 17. The method according to claim 8, wherein radar pulses constitute linearly frequency modulated segments.
 18. The method according to claim 8, wherein the periodicity of the radio frequency interference signal corresponds to the frame periodicity or line periodicity of a television signal. 